1. Field of the Invention
The present invention relates to an ink jet head and a method of producing the ink jet head.
2. Description of Related Art
Japanese patent publication No. 3,070,625 discloses an ink jet printer that includes piezoelectric actuators, a diaphragm, and a plurality of ink chambers. The piezoelectric actuators are mechanically connected to the diaphragm at positions that correspond to the ink chambers. The piezoelectric actuators serve as a drive source by extending or contracting to produce a displacement at positions corresponding to the ink chambers. The displacement generates a pressure fluctuation in the corresponding pressure chamber to eject ink from the nozzle connected to the pressure chamber.
Elongated islands are deposited on the diaphragm. Each island is positioned in between one of the piezoelectric actuators and the corresponding ink chamber. The islands are for ensuring that the piezoelectric actuators apply pressure to the diaphragm across a uniform surface area. Because the pressed surface area is the same for all ink chambers, the resolution of printed images is quite high. Also, the islands enable providing a great number of nozzles (ink chambers and piezoelectric actuators) in a small area.
The diaphragm is produced using nickel electroforming. However, nickel is relatively reactive material and so can corrode in ink. To prevent the nickel from corroding, recently a diaphragm with a two-layer structure of resin and metal has been considered. A thin metal plate is laminated onto polyethylene terephthalate, polyimide, or other resin with good chemical resistance. The metal plate is then etched to form islands at positions corresponding to where the ink chambers will be located. The side made from the resin layer confronts the ink chambers and the side with the nickel islands faces away from the ink chambers. In this way, only the resin layer is brought into contact with the ink and the nickel islands are isolated from the ink by the resin layer. Therefore, the nickel islands are not corroded.
However, resin has a large thermal expansion coefficient. The islands can be shifted out of the center of the ink chambers if the resin layer of the diaphragm expands when the diaphragm is adhered to the ink chamber structure. This is especially a problem when the ink chamber structure is made from a material with low thermal expansion. Silicon is one such low thermal expansion material that has been drawing attention because it can be etched with high precision of +/−2 microns. A complicated adhesion process must be performed to insure that the islands are located at the center of the ink chambers.
To reduce the complication of the adhesion process, it is conceivable to use an adhesive that cures at a low temperature to adhere the diaphragm to the ink chamber member. However, adhesives that cure at low temperatures of about 60° C. take a long time to harden. Efficiency of the ink jet head production process would suffer. Also, limits are placed to the types of adhesive that can be used. This also places restrictions on the ambient temperature that the ink jet printer can be used in and the types of ink that can be used in the ink jet printer.
Using the method of etching to form the islands can be problematic in a head with a highly dense nozzle arrangement of 75 dpi (dots per inch) or greater. For example, it is difficult to form the islands with proper dimensional precision because the islands have such a narrow width. Also, the islands can be unintentionally removed while forming the islands using etching. This can reduce production yield.
These problems of poor dimensional precision and removing the islands can be resolved by forming the metal islands with only a thin thickness above the surface of the resin layer. If the nickel layer is formed thin in the first place, then the etching time can also be reduced. However, when the islands are formed too thin, they do not properly perform their function because they can follow the vibration of the diaphragm plate.
U.S. Pat. No. 4,751,774 discloses adhering a molded protrusion onto the tip of each piezoelectric actuator. However, if the ink jet head has a highly dense nozzle arrangement of 75 dpi or more, then it can be quite difficult to adhere the molded protrusion members onto the tips of the piezoelectric actuators. Further, it is virtually impossible to position the protrusion members precisely at the locations of the ink chambers.